Description of the Prior Art
DE-A-43 29 155 has already disclosed a method of holding the arc on the target, i.e., on the cathode, by means of a magnetic field running approximately parallel to the surface of a PVD target and thereby achieving improved, namely uniform, evaporation of the target material.
WO-A-00/56946 discloses a generic apparatus with which material can be applied to an object by means of a physical method in vacuum for the purposes of coating an object (substrate) or a plurality of substrates. Here the objects (substrates) are located in a vacuum chamber, as described therein. In this patent application reference is made to WO-A-00/56946 and the application WO-A-00/56946 is included in its full extent in this application text.
In the coating chamber there are arranged one or several substantially cylindrical cathodes and the deposition of material is controlled with the aid of magnetic sources. The magnetic sources are preferably arranged in the cathodes and the cathode is turned relative to the magnetic field source or the magnetic field source is turned relative to the cathode to achieve uniform removal of material from the target. This material is then deposited on the substrates.
In such a chamber substrates are usually placed on a rotary table and successively coated as the table is turned. According to the prior art the cathode is placed, for example, at the centre of the rotary table so that several objects can be coated in one chamber. However, it has already been disclosed in WO-A-00/56946 that the cathode—in the case described therein, two cathodes—can rather be arranged at the edge of the vacuum chamber with the rotary table remaining at the centre of the chamber.
For instance, TiN compounds can be used for coating whereby titanium is removed from a metallic titanium target (cathode) in a nitrogen atmosphere by means of an arc and the TiN thus formed is applied to the object (substrate) to be coated by means of a voltage difference between the cathode and the substrate.
Usually here the actual coating process is preceded by etching processes. First the chamber is cleaned using an inert gas atmosphere, according to the known prior art for coating methods, wherein argon can be used as the inert gas for example. In such a process, metallic titanium, not TiN, is separated from the cathode. However, since this process is for cleaning and not for coating the substrate, it is desirable if at least part of the metallic titanium thus separated does not reach the substrate.
There is also the problem that at the beginning of the coating process larger particles are released from the target (so-called droplets). These larger particles also should not reach the substrate.
In addition, when objects are coated in such a chamber or in another chamber according to the prior art, the problem of non-uniform coating thickness arises. This is because during such coating processes the efficiency of the deposition apparatus (magnetic source and cathode) differs in relation to the height. Without special measures the coating thickness at the centre, relative to the height, is therefore greater than that at the top and at the bottom.